Various measurement appliances for determining the parameters of liquid samples are known. For example, there exist measurement appliances for determining the parameters and properties of blood samples. For example, blood gases (p02, pC02, p13), electrolytes (Na, K, Ca, Cl) and the conductivity of a sample can be determined with the aid of such measurement appliances.
In medical technology in particular, it is necessary for the measurement appliances to deliver very precise measured values of, for example, the physiological blood parameters of a patient. For this reason, the measurement appliances must be regularly checked for measurement accuracy. For quality control purposes, the entire measurement system, including its calibration media and calibration functions, is desirably inspected for correct functioning.
To check the measurement accuracy, a sample with a known content of all the analytes is measured. If the deviations between measured value and predefined value are below a certain level, this suggests correct functioning of the measurement system. If the deviations are greater than is desirable, the measurement system should not be used to measure a sample from a patient until appropriate repair or maintenance procedures have been undertaken to return it to a state in which the quality control criteria are satisfied. The quality control measurement and its results should be documented.
The reference liquid for checking the measurement accuracy is generally provided in closed ampoules which are not opened until just before use, so as to avoid contaminating or otherwise affecting the reference liquid. To ensure that the measurement results using the reference liquid are not distorted, great care must be taken when introducing the reference liquid into the measurement appliance. In the prior art, this is mainly done by the methods described below.
To remove the reference liquid from the ampoule, a syringe with an attached hollow needle is often used. The hollow needle is inserted into the ampoule in order then to draw the reference liquid by suction into the syringe barrel. When the syringe barrel has been sufficiently filled, the hollow needle can be removed from the syringe. Thereafter, the operator holds the syringe with the open end upwards and taps lightly on the syringe barrel several times so that the air, which as a result of the suction has necessarily been drawn into the reference liquid, collects in the front portion of the syringe barrel. Finally, the syringe plunger is pushed forward until the air collected in the front portion has been completely expelled. A syringe prepared in this manner can now be inserted into an input port of the measurement appliance in order to then inject the reference liquid into this. In the known method, it is also possible to do without a hollow needle.
The method described above has various disadvantages. For instance, the method takes up a great deal of the operator's time. Moreover, if the reference liquid is sucked out too sharply, there is a risk of air in the form of microbubbles getting into the reference liquid, especially if the syringe with the syringe barrel and the syringe plunger does not guarantee sufficient leaktightness. In addition, the reference liquid comes into contact with many new surfaces inside the syringe, increasing the risk of contamination of the reference liquid. Furthermore, there is no reproducibility of the conditions of introduction of the reference liquid.
To overcome these disadvantages, U.S. Pat. No. 4,275,774 describes a device for introducing a reference liquid into a measurement appliance, which device is very much like a syringe. The known device has a hollow space into which an opened ampoule is inserted and in which said inserted ampoule is locked. The known device also has a small tube arranged fixedly in the hollow chamber. The small tube reaches at one end into the inserted ampoule and at the other end leads to an outlet piece which is finally inserted into the input port of the measurement appliance. The hollow space is in communication with a pressure chamber, and the pressure in the pressure chamber can be increased or lowered with a displaceable plunger. To transfer the reference liquid into the measurement appliance, the outlet piece is inserted into the input port, and the pressure in the pressure chamber is then increased with the aid of the plunger. This pressure now acts on the surface of the reference liquid within the ampoule and drives the reference liquid through the small tube and into the measurement appliance.
The known device has the advantage that there is only a small air inclusion in the reference liquid which is introduced. Moreover, the reference liquid comes into contact with fewer new surfaces, so that there is less risk of contamination of the liquid. Nevertheless, using this device still requires a significant amount of time, and exact reproducibility of the introduction of the reference liquid into the measurement appliance is not guaranteed.
Besides the above-described methods and devices in which the pressure for introducing the reference liquid has to be applied by the operator, methods are also known in which the reference liquid is sucked in by the measurement appliance itself. Thus, for example, methods are employed in which a conduit is inserted at one end into the input port and at the other end into the ampoule, so that the measurement appliance can suck the reference liquid in through the conduit.
This method has the advantage that the reference liquid has few air inclusions and comes into contact with few new surfaces, thus largely avoiding contamination.
The pressure or underpressure applied during the introduction is also substantially constant, so that reproducibility of the transfer operations is improved. Despite these improvements, the reproducibility is still not always optimal.
To remove reference liquid from the ampoule, there is also often used a hollow needle which is implemented in the appliance. The hollow needle is inserted into the opened ampoule in order then to suck the reference liquid into the appliance. When sufficient reference liquid has been withdrawn from the ampoule, said ampoule can be detached from the hollow needle.
During the withdrawal of the reference liquid by the appliance, the operator has to hold and position the ampoule by hand via the hollow needle. The operator must ensure that the hollow needle is located within the liquid volume during suction, so as to avoid suctioning of atmospheric air. For subsequent analyses, the hollow needle implemented in the appliance must be cleaned both on the inside and on the outside in order to remove any liquid residues. The above method has various disadvantages. Thus, this method takes up a great deal of the operator's time. During the withdrawal of the liquid, the operator positions the ampoule and the hollow needle. The introduction operation, being specific to the individual user, reduces the reproducibility of the conditions of introduction of the reference liquid.
Moreover, positioning by hand entails the risk of introducing air bubbles instead of the reference liquid into the appliance. The hollow needle means that cleaning is necessary both on the inside and outside after withdrawal of the reference liquid. In the subsequent analyses, liquid residues lead to false results. As the cleaning operation is individual to the user, the reproducibility of the subsequent analyses is reduced.
Another conventional method involves withdrawing the reference liquid without the assistance of the operator. A storage receptacle holding a large number of ampoules or pouches filled with reference liquid is located in or on the appliance. To withdraw the reference liquid from the ampoule or the pouch, these are opened destructively. The introduction of the reference liquid into the appliance is preferably done by suctioning the reference liquid and conveying it through a liquid transport system. The withdrawal of the reference liquid is controlled automatically by the appliance.
This method also has various disadvantages. On account of the storage receptacle used, individual preparation for the measurement of the reference liquid is not guaranteed. Individual preparations are required, for example, as a result of different storage conditions. Parameters of influence are temperature and the state of equilibrium of the reference liquid and surrounding gas atmosphere.
The fluid system installed permanently in the appliance requires cleaning of the liquid-conveying channels after each withdrawal of reference liquid. Liquid residues lead to false measured results in the subsequent analyses.
A further disadvantage also arises from the fluid system installed permanently in the appliance. The fixed positioning of the ampoules or pouches results in very long paths of transport of the fluid system. The partial pressure measured values may be distorted by atmospheric air. In addition, the reference liquid comes into contact with many new surfaces inside the fluid system, thus increasing the risk of contamination of the reference liquid. Thus, there is a need for a device for introducing a reference liquid into a measurement appliance, and also a suitable measurement appliance, where introduction of the reference liquid into the measurement appliance can at all times be carried out under approximately identical conditions. There is also a need for a method for introducing a reference liquid into a measurement appliance for determining the parameters of liquid samples, such that the introduction of the reference liquid can at all times be carried out under approximately identical conditions.